Categories

Meta

Month: September 2010

A welder works on a ‘reentry cone’ used to guide drill pipe into the ocean floor. – IODP

Marine geologists have returned from two months at sea off British Columbia, Canada, where they installed two observatories in the ocean floor to run innovative experiments at the bottom of the sea.

The Integrated Ocean Drilling Program (IODP) “Juan de Fuca Ridge-Flank Hydrogeology” expedition–Expedition 327–left Victoria, Canada, on July 9th and returned on September 5th.

Using the scientific research vessel JOIDES Resolution, the team drilled boreholes 530 meters deep (nearly 1,800 feet) into the ocean floor to install the observatories.

“The ocean crust is the largest aquifer on the planet,” says expedition co-chief scientist Andy Fisher of the University of California at Santa Cruz. “We know it’s made up of many sections, but we have no idea how these parts connect or how they interact with one another. The observatories will help us find answers.”

These observatories, known as “CORKs” (because they are used to seal boreholes), were installed 200 kilometers (125 miles) west of Vancouver Island, Canada.

Each CORK is packed with scientific instruments that collect samples and data at multiple depths to learn more about the water, pressures, temperatures, chemistry, and microbiology within the rocks and sediments of the ocean crust.

“Expedition 327 has completed some of the most complex borehole observatory installations ever attempted,” says Jamie Allan, program director in the National Science Foundation’s (NSF) Division of Ocean Sciences, which funds IODP.

“These observatories will measure directly, within the oceanic crust, key characteristics that govern an unseen, remote, yet geographically widespread biological world, and will support long-term chemical and biological sampling and environmental monitoring of this exotic habitat.”

The CORKs are being used as part of a sampling and monitoring network to allow scientists to determine the properties of the ocean crust, and to better understand how water, heat, and chemicals are transported across vast distances below the bottom of the ocean.

The volume of salt water in the ocean crust is comparable to the volume of fresh water in Earth’s ice caps and glaciers – about 20-30 million cubic kilometers.

For comparison, this is about 2,000 times greater than the global fresh water supply, and about a half million times greater than annual fresh water usage in the U.S.

Like fresh water on land, the salt water below the seafloor is in motion, moving rapidly from place to place.

Until now, scientists have never been able to “tag” water in one place below the seafloor and determine where it flows.

Experiments begun during the expedition will provide the first direct evidence of active flow pathways and rates in the ocean crust.

Researchers used the boreholes to run experiments during the expedition. In one experiment, they injected benign tracers into the ocean floor to track the directions, rates, and patterns of fluid flow within the seafloor.

In another experiment, microbiologists placed chips of rocks and minerals in the CORKs to identify microorganisms living in the seafloor.

“It’s like lowering an empty hotel into the borehole,” explained co-chief scientist Takeshi Tsuji of Kyoto University, Japan. “When the chips are recovered in a few years, we will learn who moved in.”

Scientists estimate that a large fraction of life on Earth thrives in the “subsurface biosphere.”

Once identified, the microorganisms from the CORKs will be matched to pressure and temperature data to determine the physical conditions that are most favorable to life at different depths.

Fisher and his team will recover CORK samples and data and run additional experiments next summer and in later years.

“Through monitoring and experiments with CORKS, we will learn how microorganisms may have developed on Earth, which offers insight into how life may develop on other planets,” he says.

“We’ll also learn how carbon is transported and might be stored within deep reservoirs.”

“The results will be relayed in real time via cables as part of the Neptune Canada Observatory Network, showing the great scientific overlap between ocean observing and scientific ocean drilling.”

Three educators, an engineering student, a computer graphics animator, and an artist from the U.S. and France joined Expedition 327 to develop tools to share the expedition’s goals with non-academic audiences.

Victoria, Canada, Port Call Activities, September 5-9, 2010

The JOIDES Resolution will be in port at Ogden Point from September 5-9th, 2010. The public is invited to an afternoon of talks on September 7th from 4-6pm at the Inner Harbour Marriott, 728 Humboldt Street, Victoria.

The 25-minute talks will focus on current research in scientific ocean drilling and ocean observatories. The speakers include scientists Earl Davis (Pacific Geoscience Center, Geological Survey of Canada) and Michael Riedel (Pacific Geoscience Center, Geological Survey of Canada).

Next IODP Expedition Will Install CORK off Vancouver Island, Canada

The JOIDES Resolution embarks on its next expedition on September 9th. Led by Earl Davis of Canada’s Pacific Geoscience Center, the “Cascadia ACORK” expedition will install a new CORK observatory in the Cascadia subduction zone, about 75 kilometers off the coast of Vancouver Island. The CORK will monitor changes in pressure associated with this seismically active setting, and help scientists understand the formation of gas hydrates–ice-like deposits of gas commonly found below the ocean floor.

In a year, the new CORK will be connected to the NEPTUNE-Canada deep-ocean cable network, which will provide power and real-time data collection over the coming decades. An onboard education program, “School of Rock,” will teach 20 educators about marine geoscience. Davis and colleagues will return to Victoria on September 19, 2010.

Collier Glacier in the Oregon Cascade Range once filled this valley – note marks on North Sister, at left, from its maximum size more than 100 years ago. It’s now shrunk to less than half of its previous mass. (Photo courtesy of Oregon State University)

An Oregon State University research program has returned to Collier Glacier for the first time in almost 20 years and found that the glacier has decreased more than 20 percent from its size in the late 1980s.

The findings are consistent with glacial retreat all over the world and provide some of the critical data needed to help quantify the effects of global change on glacier retreat and associated sea level rise.

Flowing down the flanks of the Three Sisters in the central Oregon Cascade Range, Collier Glacier is at an elevation of more than 7,000 feet. It’s one of the largest glaciers in Oregon and is on a surprisingly short list – maybe 100 in the entire world – of glaciers that have been intensively studied and monitored for extended periods of time.

Glacier monitoring is difficult, dangerous and labor-intensive, OSU researchers say, and the current work, supported by the National Science Foundation, is showing an ice mass that by now has shrunk to about half of its peak size in the 1850s, when it once was nearly two miles long. Monitoring has been aided by records from early Oregon mountaineering clubs, particularly the Mazamas, founded in 1894 on the summit of Mount Hood.

A research program that began last year and is continuing this summer is now finding some rocks that are being exposed to daylight for the first time in thousands of years.

“Glaciers can tell us a lot about climate change, because they respond to both changes in temperature and precipitation,” said Peter Clark, an OSU professor of geosciences who conducted the last studies on Collier Glacier in the late 1980s and early 1990s. “They are like a checking account where you make both deposits and withdrawals, and can see the long-term effects of climate change, through the year-to-year variation in the balance between the two.”

The studies on Collier Glacier are now being conducted by Cody Beedlow, an OSU graduate student working with Clark, who visits the glacier throughout the year. Beedlow and assistants have packed in an automatic weather station that provides data on temperature, humidity, and short- and long-wave radiation. Other studies are made by drilling into the ice and inserting stakes to measure the amount of melting.

“Even to get up onto the glacier in the summer you need to travel pretty fast and light, before the next storm front moves in,” Beedlow said. “We usually start hiking in at night with headlamps, and often get off the glacier just as the clouds are piling up. For this kind of science you have to take your opportunities when you can find them.”

The research in the 1980s and 90s showed the glacier losing mass in four of the five years studied, and it also lost mass last year, Beedlow said. Researchers have been able to get in to the glacier earlier in the season than they had previously, he said, providing important new data.

The glaciers in the Pacific Northwest, such as Collier and another large ice mass on Mount Hood, Eliot Glacier, are there primarily because of massive winter snowfall – more than 20 feet at times on the Three Sisters – which does not all melt during the summer. Elsewhere in the world where it’s much colder, such as Antarctica, there’s very little snowfall but the temperature is so cold that snowfall remains almost permanently.

But in most of the world, including the Pacific Northwest, glaciers have been in a slow global retreat since the end, in the late-1800s, of a 600-year period called the “Little Ice Age,” Clark said. Some of that melting will cause a noticeable increase in sea level, and some water resources will be affected where glacial fields feed irrigation streams and reservoirs.

“There will be some ecological and agricultural impacts from glacier loss,” Clark said. “But from our perspective, studies such as what we’re doing on Collier Glacier give very valuable information to help understand past and current climate changes. They are very good barometers of climate effects.”

Long-term studies of Collier Glacier, through scientific research and observations made by examining old photographs, suggest it’s now about half of the mass it was 150 years ago. It appears to lose mass most quickly during El Nino events, and also had a period of rapid decline from 1924-34.

Some of the locations where researchers now camp would have been several hundred feet deep in ice in the 1800s.

This is an image of a sample of cratonic mantle root from Kimberley, South Africa. The rock consists of dark green olivine, whitish-green enstatite, emerald green diopside and purple garnet. – Credit: David R. Bell / ASU

Earth today is one of the most active planets in the Solar System, and was probably even more so during the early stages of its life. Thanks to the plate tectonics that continue to shape our planet’s surface, remnants of crust from Earth’s formative years are rare, but not impossible to find. A paper published in Nature Sept. 2 examines how some ancient rocks have resisted being recycled into Earth’s convecting interior.

Throughout the world there exist regions of ancient crust, referred to as cratons, which have resisted being recycled into the interior of our tectonically dynamic planet. These geologic anomalies appear to have withstood major deformation thanks to the presence of mantle roots. A mantle root is a portion of Earth’s mantle that lies beneath the craton, extending like the root of a tooth into the rest of the underlying mantle.

Just like a tooth, the mantle root of a craton is compositionally different from the normal mantle into which it protrudes. It is also colder, causing it to be more rigid. These roots were formed in ancient melting events and are intrinsically more buoyant than the surrounding mantle. The melting removed much of the calcium, aluminum, and iron that would normally form dense minerals. Thus, these roots act as rafts bobbing on a vigorously convecting mantle, on which old fragments of continental crust may bask in comparative safety.

However, geophysical calculations have suggested that this buoyancy is not enough to stop destruction of the mantle roots. According to these calculations, the hotter temperatures that are widely thought to have existed in Earth’s mantle about 2.5 to 3 billion years ago should have warmed and softened up the base of these roots sufficiently to allow them to be gradually eroded from below, leading to their eventual destruction as they were entrained, piece by piece, into the convecting mantle. A stronger viscosity contrast between the root and the underlying mantle is required to ensure preservation.

In the Sept. 2 issue of Nature, Anne Peslier, an ESCG-Jacobs Technology scientist working at NASA-Johnson Space Center and her colleagues David Bell from Arizona State University and Alan Woodland and Marina Lazarov from the University of Frankfurt, published measurements of the trace water content of rocks from the deepest part of a mantle root that offer an explanation for this mystery.

“It has long been suspected, but not proven, that cratonic mantle roots are dryer than convecting upper mantle,” explains Bell, an associate research scientist in the School of Earth and Space Exploration and the department of chemistry and biochemistry in ASU’s College of Liberal Arts and Sciences. “The presence of very small quantities of water is known to weaken rocks and minerals. During partial melting, such as that experienced by the mantle roots, water – like calcium, aluminum and iron – is also removed.”

The researchers used samples found in diamond mines of Southern Africa, where the ancient crust of the Kaapvaal craton was pierced about 100 million years ago by gas-charged magmas called kimberlites. These magmas were generated at depths of about 125 miles (200 kilometers) beneath the mantle root and ascended rapidly (in a matter of hours) through the Earth via deep fractures, bringing with them pieces of the rocks traversed, including diamonds. After erupting explosively at the surface, the magmas solidified into the pipe-like bodies of kimberlite rock that were subsequently mined for their diamonds.

The mantle rocks analyzed by the team were transported from a range of depths down to 125 miles (200 km) below the surface, where they had resided since their formation around 3 billion years ago. The samples of rock called peridotite are composed mainly of the mineral olivine, with minor quantities of pyroxenes and garnet. Olivine is, because if its abundance, the mineral believed to control the rheological properties of peridotite.

What Peslier and colleagues found is that beyond a depth of about 112 miles (180 km), the water content of olivines begins to decline with depth, so that the olivine in peridotite samples from the very base of the cratonic mantle root contained hardly any water. That makes these olivines very hard to deform or break up, and may generate the strong viscosity contrast with that geophysical models of craton root stability require.

Why the bottom of the mantle root has dry olivines is still a matter of speculation. One possibility, suggested by Woodland, is that reducing conditions thought to prevail at these depths would ensure that fluids would be rich in methane instead of water. Bell suggests that melts generated in the asthenosphere, such as those eventually giving rise to kimberlite eruptions, may scavenge any water present while passing through the base of the cratonic root and transport it into the overlying shallower mantle.

These results reiterate the belief shared by many scientists that knowing how much water is present deep in terrestrial planets and moons, like Earth, Mars or the Moon, is important to understanding their dynamics and evolutionary history.

ESA’s Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland’s Petermann glacier on August 4, 2010. This animation, generated from 24 Envisat Advanced Synthetic Aperture Radar images acquired from July 31 to September 1, shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait — a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay. – ESA

ESA’s Envisat satellite has been tracking the progression of the giant iceberg that calved from Greenland’s Petermann glacier on 4 August 2010. This animation shows that the iceberg, the largest in the northern hemisphere, is now entering Nares Strait – a stretch of water that connects the Lincoln Sea and Arctic Ocean with Baffin Bay.

The Petermann glacier in northern Greenland is one of the largest of the country’s glaciers – and until August it had a 70 km tongue of floating ice extending out into the sea. The glacier regularly advances towards the sea at about 1 km per year.

Earlier this year, satellite images revealed that several cracks had appeared. Envisat radar images showed that the ice tongue was still intact on 3 August but, on 4 August, a huge chunk had detached.

Calvings from the Petermann glacier are quite common, but one of this magnitude is rare. Less significant events took place in 2001, in 2008 when a 27 sq km iceberg made its way south to Davis Strait, and in 2009.

This iceberg is about 30 km long and 15 km wide at its foot and almost 7 km wide at its head, covering an area of around 245 sq km. By 22 August this giant mass of ice had been carried about 22 km from its birth place.

On 1 September imagery showed that the iceberg had travelled almost another 6 km from the edge of the glacier and rotated westward (about 39°), just tipping into Nares Strait. The animation also shows that the iceberg hit a small island, which may delay further progression for a short while and may also cause the iceberg to break.

It is expected that the iceberg will soon be fully in Nares Strait, but its course depends on winds blowing off the glacier and currents in the strait, as well as sea ice that could block its path.

Using a diamond-anvil cell to recreate the high pressures deep within the earth, researchers at the California Institute of Technology (Caltech) have found unusual properties in an iron-rich magnesium- and iron-oxide mineral that may explain the existence of several ultra-low velocity zones (ULVZs) at the core-mantle boundary. A paper about their findings was published in a recent issue of Geophysical Research Letters (GRL).

ULVZs-which were first discovered in the early 1990s by researchers at Caltech led by Donald V. Helmberger, Smits Family Professor of Geological and Planetary Sciences-are found in a patchwork distribution just above the core-mantle boundary, which is located at a depth of 2,900 kilometers. In the ULVZs, which range from a few kilometers to tens of kilometers in depth and are up to 100 kilometers across, the velocities of seismic waves slow down by up to 30 percent.

Previously, geophysicists had suggested that the ULVZs might be composed of liquid-bearing, partially melted materials; in this region of the lower mantle, the earth is solid-but plastic and flowing-rock. The idea was that if these rocks contained some liquid, seismic waves would propagate more slowly through them. And, indeed, “the area is very hot and close to the core,” says Jennifer M. Jackson, assistant professor of mineral physics at Caltech and coauthor of the GRL paper. The catch, she says, “is that the temperature and composition of this region are not very well known.” In addition, she says, “these ULVZs are not always associated with surface hot spots” such as the Hawaiian islands.

Instead of being patches of partially melted (i.e., liquid-bearing) rock, the ULVZs-Jackson and her colleagues believe-are composed of an entirely solid, compositionally distinctive rock type with unusual properties that cause sound waves to slow down. “What we’re suggesting here is that many ULVZs-it need not be all of them-are likely to be solid, not molten,” Jackson says.

In reaching this conclusion, Jackson and her team studied the properties of iron-rich magnesium-iron oxides [(Mg, Fe)O]-similar to the mineral periclase known at the earth’s surface-using specially prepared diamond-anvil cells. Within the piston-like chamber of the 4-inch-tall cell, two semiflawless natural diamonds-a quarter of a carat each-were squeezed together, sandwiching a small sample of the oxide and proportionally increasing its pressure.

After the pressurized samples were created, they were taken to the Advanced Photon Source at Argonne National Laboratory in Illinois and exposed to X-rays, causing the scattering of photons at energies related to the speed at which sound would travel through.

Jackson and her colleagues conducted the measurements at pressures ranging from ambient pressure (at Earth’s surface) up to 121 Gigapascals (GPa)-or over 17 million pounds per square inch, equivalent to about 2700 km depth. “The measurements stopped at pressures where the diamond-anvils broke,” Jackson says.

Normally, solid materials increase in stiffness under increasing pressure, causing sound waves to travel at higher and higher velocities. But in iron-rich (Mg,Fe)O, the sound velocities took a surprising and significant dip of about 10 percent at just under 28 GPa. The sound velocities in the minerals did not return to their ambient-pressure levels until pressures of 50 to 60 GPa were reached.

To their surprise, the researchers found they could compress the iron-rich mineral to very high pressures and very high densities, “and yet it is still highly compressible” Jackson says. In fact, even at 121 GPa, sound velocities in the mineral were still much lower than in other known mantle materials. “It’s quite unusual to have a solid this compressible under these pressures. Compared to silicate”-the main constituent of Earth’s crust-“at the same pressures, it’s like squishing a pat of butter between two bricks.”

Jackson and her colleagues suspect that the velocity drops in this particular mineral are related to magnetic transitions that can make iron-rich oxides more compressible than many silicates. “Silicate acts more like a very stiff spring; iron-rich oxide is like a very weak spring,” she explains. With increasing pressure, “the iron-oxide goes through a series of complex magnetic transitions that couple to its sound waves”-and thus explain the unusually low velocities, even at extremely high pressures. “But,” she adds, “because the mineral is so iron-rich, it is likely only to exist near the core.”

While the new result does not rule out partial melting as a cause of some ULVZs, Jackson says, “iron-rich periclase certainly provides one of the most robust explanations so far.”

“The composition of the boundary layer between the earth’s liquid outer core and silicate mantle not only influences the current thermal and chemical evolution of the earth’s interior, but may hold the key to unlocking the planet’s past thermo-chemical evolution,” she says. “The iron-rich periclase patches may be fossil remnants of large-scale melting events occurring billions of years ago inside the earth. Having an iron-rich oxide in contact with the earth’s core would facilitate propagation of the planet’s magnetic field and contribute to the dynamic coupling between the core and the mantle.”

Tyrone Daulton is pictured with the transmission electron microscrope he used to search in vain for shock-synthesized nanodiamonds, evidence that a extraterrestrial object such as a meteorite killed off North American megafauna. – Tyrone Daulton

About 12,900 years ago, a sudden cold snap interrupted the gradual warming that had followed the last Ice Age. The cold lasted for the 1,300-year interval known as the Younger Dryas (YD) before the climate began to warm again.

In North America, large animals known as megafauna, such as mammoths, mastodons, saber-tooth tigers and giant short-faced bears, became extinct. The Paleo-Indian culture known as the Clovis culture for distinctively shaped fluted stone spear points abruptly vanished, eventually replaced by more localized regional cultures.

What had happened?

One theory is that either a comet airburst or a meteor impact somewhere in North America set off massive environmental changes that killed animals and disrupted human communities.

In sedimentary deposits dating to the beginning of the YD, impact proponents have reported finding carbon spherules containing tiny nano-scale diamonds, which they thought to be created by shock metamorphism or chemical vapor deposition when the impactor struck.

The nanodiamonds included lonsdaleite, an unusal form of diamond that has a hexagonal lattice rather than the usual cubic crystal lattice. Lonsdaleite is particularly interesting because it has been found inside meteorites and at known impact sites.

In the August 30 issue of the Proceedings of the National Academy of Sciences, a team of scientists led by Tyrone Daulton, PhD, a research scientist in the physics department at Washington University in St. Louis, reported that they could find no diamonds in YD boundary layer material.

Daulton and his colleagues, including Nicholas Pinter, PhD, professor of geology at Southern Illinois University In Carbondale and Andrew C. Scott, PhD, professor of applied paleobotany of Royal Holloway University of London, show that the material reported as diamond is instead forms of carbon related to commonplace graphite, the material used for pencils.

“Of all the evidence reported for a YD impact event, the presence of hexagonal diamond in YD boundary sediments represented the strongest evidence suggesting shock processing,” Daulton, who is also a member of WUSTL’s Center for Materials Innovation, says.

However, a close examination of carbon spherules from the YD boundary using transmission electron microscopy by the Daulton team found no nanodiamonds. Instead, graphene- and graphene/graphane-oxide aggregates were found in all the specimens examined (including carbon spherules dated from before the YD to the present). Importantly, the researchers demonstrated that previous YD studies misidentified graphene/graphane-oxides as hexagonal diamond and likely misidentified graphene as cubic diamond.

The YD impact hypothesis was in trouble already before this latest finding. Many other lines of evidence – including: fullerenes, extraterrestrial forms of helium, purported spikes in radioactivity and iridium, and claims of unique spikes in magnetic meteorite particles – had already been discredited. According to Pinter, “nanodiamonds were the last man standing.”

“We should always have a skeptical attitude to new theories and test them thoroughly,” Scott says, “and if the evidence goes against them they should be abandoned.”

Inorganic elements known to be toxic at low concentrations are being discharged to air and water by oilsands mining and processing according to University of Alberta (U of A) research findings being published this month in one of the world’s top scientific journals.

The 13 elements being discharged include mercury, arsenic, lead, cadmium and several other metals known to be toxic at trace levels. The paper will appear in the August 30 edition of the Proceedings of the National Academy of Sciences (PNAS).

The results are not surprising according to corresponding author David Schindler – an internationally acclaimed researcher in the Department of Biological Sciences at the U of A – given the huge amounts of many of the same elements that the industry has reported discharging, according to Environment Canada’s National Pollutant Release Inventory.

“Given the large amounts of pollutants released, any monitoring program that cannot detect increases in the environment must be considered as incompetent,” says Schindler, referring to the Regional Aquatic Monitoring Program.

“The U of A study was deliberately designed to test claims by industry and Alberta politicians that all contaminants in the river are from natural sources,” said Schindler.

This included examining patterns of deposition of pollutants in snow and releases to water both near to, and remote from, industry.

“Rather than pollutants increasing continuously downstream in the river due to natural sources, as government has claimed, concentrations of the majority of toxins were always highest near sites of industrial activity,” Schindler says.

He notes however that concentrations of many contaminants remained above background levels right up to the Athabasca Delta. Elevated concentrations were in Lake Athabasca, near Fort Chipewyan.

“The releases are in clear violation of section 36, subsection 3 of the Fisheries Act, which prohibits discharge of toxins in any quantity into fish-bearing waters.”

Schindler says much of the debate over the impact of oilsands has been based on a combination of conjecture and propaganda, which has not been peer reviewed or published in recognized scientific publications.

An earlier (December, 2009) paper by the research group documented the release by the oilsands industry of a number of organic carcinogens, similar to those released by the BP spill into the Gulf of Mexico, and the Exxon Valdez into the Gulf of Alaska.

This cross section shows the adaptively refined mesh with a finest resolution of about 1 km in the region from the New Hebrides to Tonga in the SW Pacific. The refinement occurs both around plate boundaries and dynamically in response to the nonlinear rheology. – Georg Stadler, Institute for Computational Engineering & Sciences, UT Austin

Computational scientists and geophysicists at the University of Texas at Austin and the California Institute of Technology (Caltech) have developed new computer algorithms that for the first time allow for the simultaneous modeling of the earth’s Earth’s mantle flow, large-scale tectonic plate motions, and the behavior of individual fault zones, to produce an unprecedented view of plate tectonics and the forces that drive it.

A paper describing the whole-earth model and its underlying algorithms will be published in the August 27 issue of the journal Science and also featured on the cover.

The work “illustrates the interplay between making important advances in science and pushing the envelope of computational science,” says Michael Gurnis, the John E. and Hazel S. Smits Professor of Geophysics, director of the Caltech Seismological Laboratory, and a coauthor of the Science paper.

To create the new model, computational scientists at Texas’s Institute for Computational Engineering and Sciences (ICES)-a team that included Omar Ghattas, the John A. and Katherine G. Jackson Chair in Computational Geosciences and professor of geological sciences and mechanical engineering, and research associates Georg Stadler and Carsten Burstedde-pushed the envelope of a computational technique known as Adaptive Mesh Refinement (AMR).

Partial differential equations such as those describing mantle flow are solved by subdividing the region of interest (such as the mantle) into a computational grid. Ordinarily, the resolution is kept the same throughout the grid. However, many problems feature small-scale dynamics that are found only in limited regions. “AMR methods adaptively create finer resolution only where it’s needed,” explains Ghattas. “This leads to huge reductions in the number of grid points, making possible simulations that were previously out of reach.”

“The complexity of managing adaptivity among thousands of processors, however, has meant that current AMR algorithms have not scaled well on modern petascale supercomputers,” he adds. Petascale computers are capable of one million billion operations per second. To overcome this long-standing problem, the group developed new algorithms that, Burstedde says, “allows for adaptivity in a way that scales to the hundreds of thousands of processor cores of the largest supercomputers available today.”

With the new algorithms, the scientists were able to simulate global mantle flow and how it manifests as plate tectonics and the motion of individual faults. According to Stadler, the AMR algorithms reduced the size of the simulations by a factor of 5,000, permitting them to fit on fewer than 10,000 processors and run overnight on the Ranger supercomputer at the National Science Foundation (NSF)-supported Texas Advanced Computing Center.

A key to the model was the incorporation of data on a multitude of scales. “Many natural processes display a multitude of phenomena on a wide range of scales, from small to large,” Gurnis explains. For example, at the largest scale-that of the whole earth-the movement of the surface tectonic plates is a manifestation of a giant heat engine, driven by the convection of the mantle below. The boundaries between the plates, however, are composed of many hundreds to thousands of individual faults, which together constitute active fault zones. “The individual fault zones play a critical role in how the whole planet works,” he says, “and if you can’t simulate the fault zones, you can’t simulate plate movement”-and, in turn, you can’t simulate the dynamics of the whole planet.

In the new model, the researchers were able to resolve the largest fault zones, creating a mesh with a resolution of about one kilometer near the plate boundaries. Included in the simulation were seismological data as well as data pertaining to the temperature of the rocks, their density, and their viscosity-or how strong or weak the rocks are, which affects how easily they deform. That deformation is nonlinear-with simple changes producing unexpected and complex effects.

“Normally, when you hit a baseball with a bat, the properties of the bat don’t change-it won’t turn to Silly Putty. In the earth, the properties do change, which creates an exciting computational problem,” says Gurnis. “If the system is too nonlinear, the earth becomes too mushy; if it’s not nonlinear enough, plates won’t move. We need to hit the ‘sweet spot.'”

After crunching through the data for 100,000 hours of processing time per run, the model returned an estimate of the motion of both large tectonic plates and smaller microplates-including their speed and direction. The results were remarkably close to observed plate movements.

In fact, the investigators discovered that anomalous rapid motion of microplates emerged from the global simulations. “In the western Pacific,” Gurnis says, “we have some of the most rapid tectonic motions seen anywhere on Earth, in a process called ‘trench rollback.’ For the first time, we found that these small-scale tectonic motions emerged from the global models, opening a new frontier in geophysics.”

One surprising result from the model relates to the energy released from plates in earthquake zones. “It had been thought that the majority of energy associated with plate tectonics is released when plates bend, but it turns out that’s much less important than previously thought,” Gurnis says. “Instead, we found that much of the energy dissipation occurs in the earth’s deep interior. We never saw this when we looked on smaller scales.”

<IMG SRC="/Images/912348330.jpg" WIDTH="350" HEIGHT="309" BORDER="0" ALT="This graphic shows the thickness (in kilometers) of the North American lithosphere. The blue area is about 250 km thick and, based on new findings reported in Nature, is composed of a 3-billion-year old craton underlain by younger lithosphere deposited as ocean floor subducted under the continent within the past billion years. The green, yellow and red areas are younger and thinner continental lithosphere added around the margins of the original craton, also by subducting sea floor. The thick broken line indicates the borders of the stable part of the continent. – Barbara Romanowicz and Huaiyu Yuan, UC Berkeley”>

This graphic shows the thickness (in kilometers) of the North American lithosphere. The blue area is about 250 km thick and, based on new findings reported in Nature, is composed of a 3-billion-year old craton underlain by younger lithosphere deposited as ocean floor subducted under the continent within the past billion years. The green, yellow and red areas are younger and thinner continental lithosphere added around the margins of the original craton, also by subducting sea floor. The thick broken line indicates the borders of the stable part of the continent. – Barbara Romanowicz and Huaiyu Yuan, UC Berkeley

The North American continent is not one thick, rigid slab, but a layer cake of ancient, 3 billion-year-old rock on top of much newer material probably less than 1 billion years old, according to a new study by seismologists at the University of California, Berkeley.

The finding, which is reported in the Aug. 26 issue of Nature, explains inconsistencies arising from new seismic techniques being used to explore the interior of the Earth, and illuminates the mystery of how the Earth’s continents formed.

“This is exciting because it is still a mystery how continents grow,” said study co-author Barbara Romanowicz, director of the Berkeley Seismological Laboratory and a UC Berkeley professor of earth and planetary science. “We think that most of the North American continent was constructed in the Archean (eon) in several episodes, perhaps as long ago as 3 billion years, though now, with the present regime of plate tectonics, not much new continent is being formed.”

The Earth’s original continents started forming some 3 billion years ago when the planet was much hotter and convection in the mantle more vigorous, Romanowicz said. The continental rocks rose to the surface – much like scum floats to the top of boiling jam – and eventually formed the lithosphere, Earth’s hard outer layer. These old floating pieces of the lithosphere, called cratons, apparently stopped growing about 2 billion years ago as the Earth cooled, though within the last 500 million years, and perhaps for as long as 1 billion years, the modern era of plate tectonics has added new margins to the original cratons, slowly expanding the continents.

“Since the Archean, the continents have been broken up in pieces, glued back together and then broken up again, but those pieces of the very old lithosphere – very old pieces of continents – have been there for a very long time,” she said.

One of those original continents is the North American craton, located mostly in the Canadian part of North America. The study suggests that what continental lithosphere has been added since the original North American craton formed was scraped off of the ocean floor as it plunged beneath the continent, not deposited from below by plumes of hot material welling up through the mantle.

The history of the Earth’s oldest continental plates is vague because details of their interiors are hidden from geologists. The top 40 km of the lithosphere is crust that is chemically distinct from the mantle below, and while activities such as mountain building can dredge up deeper material, mountain building is rare in the planet’s stable cratons. The deep interior of the North American craton is known only from so-called xenoliths – rock inclusions in igneous rock – or xenocrysts such as diamonds that have been delivered to the surface from deep below by volcanoes.

Seismologists, however, have the ability to probe the Earth’s interior thanks to seismic waves from earthquakes around the globe, which can be used much like sound waves are used to probe the interior of the human body. Such seismic tomography has established that the bottom of the North American craton is about 250 km deep at its thickest, thinning out toward the margins where new chunks have been added to the continental lithosphere. Below the rigid lithosphere is the softer asthenosphere, on which the continental and oceanic plates ride.

Romanowicz and UC Berkeley postdoctoral fellow Huaiyu Yuan are testing a new technique, seismic azimuthal anisotropy, to look for the boundary between the lithosphere and asthenosphere. The technique takes advantage of the fact that seismic waves travel faster when moving in the same direction that a rock has been stretched than when traveling across the stretch marks. The difference in speed makes it possible to detect layers that have been stretched in different directions.

“As the lithosphere moves over the asthenosphere, the material gets stretched and acquires texture, which indicates the direction in which the plates are moving,” she said.

Surprisingly, they found a sharp boundary 150 kilometers below the surface, far too shallow to be the lithosphere-asthenosphere boundary. The scientists believe that the sharp boundary is between two types of lithosphere: the old craton and the younger material that should match the chemical composition of the sea floor. Their interpretation fits with studies of xenoliths and xenocrysts, which indicate that there are two chemically distinct layers within the Archean crust.

Coincidentally, three years ago, researchers using a popular new technique called receiver function studies detected a sharp boundary below the North American craton at a depth of about 120 km. Receiver function studies take advantage of the fact that seismic waves change character – converting from a P wave to an S wave, for example – at sharp boundaries.

“We think they are seeing the same layering we are seeing, a sharp boundary within the lithosphere,” Romanowicz said.

The stretch marks revealed by azimuthal anisotropy seem to rule out one theory of how the older continents have accrued more lithosphere.

“One hypothesis was that the bottom part was formed by underplating,” Romanowicz said. “You would have a big plume of material, an upwelling, that would get stuck under the root. But what we are observing is not consistent with that. The material would spread in all directions and you would see anisotropy that is pointing like spokes in a bicycle.”

“We are seeing a very consistent direction across the whole craton. In the top lithospheric layer the fast axis is, on average, aligned northeast-southwest. In the bottom layer it is aligned more north-south. So underplating doesn’t work,” she said.

If subduction is adding to the continental lithosphere, on the other hand, the north-south strike of the subduction zones on the east and west sides of the North American craton is consistent with the direction Romanowicz and Yuan found.

“I think our paper will stimulate people to look more carefully at distinguishing the ages of the lithosphere as a function of depth,” she said. “Any information we can provide that constrains models of continental formation is really useful to the geodynamicists.”